Global positioning system (GPS) satellites transmit two coherent signals at different frequencies generated from a common time and frequency source. A receiver demodulates these signals to recover the underlying carrier frequency signal. The coherency of the two carriers facilitates the recovery of integer wavelength ambiguities in applications using carrier-based differential techniques. Many land applications of GPS (survey, ionospheric research, geophysical surface motion monitoring, etc.) use this technique. Typically, some residual ambiguity results from using the two coherent signals to resolve the integer wavelength ambiguities, and other methods are necessary to accelerate and/or confirm the accuracy of the ambiguity resolution results.
In a number of applications of carrier-based differential GPS (aircraft automatic landing and open-pit mine equipment positioning, for example), pseudolites augment the GPS satellite signals. In the landing application, the pseudolites facilitate the integer wavelength ambiguity resolution by incorporating the rapid geometry change due to the motion of the airplane close to the nearby pseudolites into the solution for the integer ambiguities. Cohen at al., U.S. Pat. No. 5,572,218, describes such an application.
In the open-pit mining application, for example, pseudolites augment the satellites obscured from view by high mine walls and cliffs. Single-frequency pseudolites used in this fashion provide additional code-phase measurements, but they do not aid in rapidly resolving carrier cycle ambiguities: The motion of the vehicles (shovels, trucks, crawlers, etc.) does not provide large geometry changes with respect to the pseudolites in a short period.
Accordingly, an object of this invention is to rapidly resolve integer ambiguity—even without significant vehicle motion relative to the pseudolites.
Another object of the invention is to enhance the integrity and speed of the pseudolite technique described above.
Still another object of the invention is to leverage conventional GPS equipment, including GPS receivers and pseudolites, to reduce the cost of a system.
These and other goals of the invention will be readily apparent to one of skill in the art on reading the background above and the description below.